The present invention is related to user-manipulated exercise equipment utilizing a weight-plate stack comprising a plurality of stacked weight plates.
The typical weight plate in the stack has a generally vertical central hole, through which a selector rod passes downwardly from a cable that couples the rod to a user-manipulated bar or handle via one or more pulleys. The selector rod typically has a series of vertically-separated, generally horizontally-extending holes that generally align with a like series of vertically-separated, generally horizontally-extending openings in the weight-plate stack, one for each plate in the stack. Each plate in the stack accordingly has a transversely-extending opening extending from the front of the weight plate to at least the central opening. A selector pin can thereby be inserted through the transverse opening associated with a selected weight in the stack, and into the aligned hole in the selector rod, so that the selected weight plate and all weight plates above it in the stack are lifted by a user via the handle/bar, selector rod and connecting cable. The amount of resistance lifted can therefore be adjusted by relocating the selector pin to a higher or lower weight in the stack.
As it travels generally vertically up and down in response to the user's exercise movement, the stack is typically guided by a guide rod system. Each weight plate in the stack has one or more guide rod holes that extend through the weight plate generally parallel to the central hole that accommodates the selector rod. The apparatus, of which the stack is a component, provides one or more generally vertically-extending guide rods that are positioned to pass through the guide rod holes of the weight plates in the stack.
Those of ordinary skill in the art will recognize that the weight plates within the stack can shift with respect to each other during movement. This generally arises because of manufacturing tolerances in the location and diameters of the plates' openings and/or holes that accommodate the guide-rod(s), the selector rod and the selector pin. The slight degrees of size and positional error can result in the plates rubbing against the guide rods, creating friction or drag that can be felt by the user. Similarly, positional errors give rise to alignment errors between the vertically-separated holes in the selector rod and the respective selector pin-accommodating holes/openings associated with in the weight plates, making it difficult and/or annoying to insert and remove the selector pin when adjusting the amount of weight to be lifted; users must sometimes even need to jiggle the handle/bar of the apparatus in order to create the momentary alignment needed to remove or insert the selector pin.
Another issue pertaining to non-aligned weight plates in the stack is particularly noticeable with “high-end” machines using stainless steel weight plates or weight plates of other reflective materials. As the plates are raised and lowered, they become slightly misaligned owing to the same manufacturing tolerances discussed above, and the stack exhibits an unattractive appearance. The problem is particularly acute with reflective materials because the human eye is very sensitive to slight differences in the angles of reflection that result from the misalignments.
Lastly, non-aligned weight plates affect the effectiveness of a workout in a profoundly subtle way. A weight-lifting exercise movement generally comprises two phases: a positive and a negative. The positive (or “concentric”) contraction is the phase in which the targeted muscle group contracts to move the weight from a first position to a second position. The negative (or “eccentric”) phase of the movement occurs as the weight is returned from the second position to the first. During the negative phase, the targeted muscle fibers lengthen, but the muscles are still working and contracting in order to control the speed at which the weight returns to the first position.
During the negative phase, a person can work against approximately 40% more weight than (s)he can during the positive phase; i.e., a person is “stronger” during the negative phase. So, for example, one is capable of lowering 140 lbs to the aforedescribed first position if one can raise 100 lbs. to the aforedescribed second position.
Assuming that non-aligned plates result in a 10% friction loss, which appears to be a reasonable estimate, a person capable of raising 110 lbs can only raise 100 lbs of weight plate mass, since friction is adding 10 lbs to the encountered resistance. However, during the negative phase, the person is only working against 100 lbs. of resistance during a phase when (s)he is capable of working against 140% of 110 lbs., or 154 lbs.
Accordingly, the non-aligned plates cause a resistance adjustment in the wrong direction during the negative phase. If friction can be eliminated, the lifter can resist 110 lbs of weight plate mass during both phases, yielding a better workout during the negative phase and a more accurate assessment of actual weight moved.
These and other details concerning the invention will be apparent from the following description of the preferred embodiment, of which the drawings form a part.